Technical Field
The present invention relates to test instruments including a passive intermodulation (PIM) measurement instrument, and in particular to a portable user interface for test instrumentation.
Related Art
A PIM is an unwanted signal or signals generated by the non-linear mixing of two or more frequencies in a passive device such as a connector or cable. PIM has surfaced as a problem for cellular telephone technologies such as Global System for Mobile Communications (GSM), Advanced Wireless Service (AWS) and Personal Communication Service (PCS) systems. Cable assemblies connecting a base station to an antenna on a tower using these cellular systems typically have multiple connectors that cause PIMs that can interfere with system operation.
The PIM signals are created when two signals from the same or different systems mix at a PIM point such as a faulty cable connector. If the generated PIM harmonic frequency components fall within the receive band of a base station, it can effectively block a channel and make the base station receiver think that a carrier is present when one is not. PIMs can, thus, occur when two base stations operating at different frequencies, such as an AWS device and a PCS device, are in close proximity.
The PIMs can be reduced or eliminated by replacing faulty cables or connectors. Test systems can be utilized to detect the PIMs enabling a technician to locate the faulty cable or connector. The test system to measure the PIMs, thus, creates signals at two different frequencies, amplifies them, and provides them through cables connecting a base station to antennas on a tower for the base stations. A return signal carrying the PIMs is filtered to select a desired test frequency harmonic where PIMs can be detected and the PIM measurement is provided to an operator.
PIM testers to date have used CW signals for the two frequencies used to create the PIM. This is due to the unknown nature of where physically the PIM is located in the transmission path. The PIM is monitored by one technician while the other technician climbs the tower and physically moves the connector joints to see if the PIM changes. Other techniques plot a time graph of the PIM so a single technician can correlate his movement up the tower with results on a graph provided on a plotter below the tower.
FIG. 1 shows a block diagram of components of a prior art test system for measuring PIM. As shown in FIG. 1, the system includes a controller 100, such as a PC, and a measurement instrument 102. In this example the measurement instrument measures PIM. The controller includes a CPU 106, Display 108 and User Input device 110. The controller is connected to the measurement instrument via interface 112. The measurement instrument provides an RF signal 114 to measurement engine 116.
In test systems of the prior art, the controller and the measurement instrument can be packaged separately and then physically connected using a common interface such as GPIB, RS232, or USB which provides a tethered form of remote control. In other test systems, the controller and the measurement instrument are packaged into a single instrument package 118. Such a test system can then be connected to whatever device or system is under test, for example it can be connected at a base station to test the base station to the antennas.
As described above, such test systems often require more than one technician to be used properly. This can be because these test systems are largely immobile once they are positioned for testing. That is, they cannot be carried with the technician as he or she climbs the tower or performs other visual inspections. Thus, one technician is left with the test system while a second technician visually and manually inspects the cables, connectors and other potential PIM sources in the surrounding environment, separating the technicians by up to several hundred meters. These multiperson systems can add time and costs to the diagnostic process. Further, although some systems enable a single technician to use them, these systems plot PIM against time. While this allows the technician to correlate his actions as he climbs a tower to the incidence of PIM on the line, it does not provide the technician with real time information because the test system cannot be used while the technician climbs the tower or surveys the surrounding area.